There are numerous components in industrial and utility installations which require on-site pre-service inspection and/or periodic in-service inspection to verify continued serviceability. Such inspection frequently includes dimensional measurements. A notable example is the on-site inspection of components of a nuclear reactor, especially replaceable components such as control rods, fuel elements and fuel channels. A typical fuel assembly having a removable and reusable flow channel 36 is shown in U.S. Pat. No. 3,689,358. Such an elongated fuel channel may be in the order of 13 cm in transverse cross section dimensions and in the order of 4 m in length. Dimensional tolerances over the length of such a channel may be in order of 0.127 mm or less. To perform the necessary measurement of such a component, it is clear that an elongated accurate and stable plane of reference is required.
Prior attempts to provide accurate and stable measurement reference planes have involved structural members with large moments of inertia and great mass. Examples are thick granite or metal surface plates and multiple beam structures. An example of the surface plate approach is shown in German patent publication No. 2,532,840. Beam structures are exemplified by U.S. Pat. Nos. 4,036,686 and 4,048,009. Such prior approaches generally have been unsatisfactory because of the difficulty of transporting such massive structures and because of distortion thereof from handling and from thermal stresses.
Thus an object of the invention is to provide a light-weight, portable and dimensionally stable structure capable of maintaining highly accurate measurement reference planes.